Aminophylline ↔ theophylline

Convert between aminophylline and theophylline using the 0.8 salt factor.

What is this for?

Aminophylline is the ethylenediamine salt of theophylline and is approximately 80% theophylline by weight. The mnemonic is ATM — Aminophylline To theophylline, Multiply by 0.8 — with division in the reverse direction.

How to use it

Select the direction of the conversion.
Enter the dose in milligrams.
Recheck a theophylline concentration after conversion — the narrow therapeutic index leaves little margin.

Worked example

A patient is receiving aminophylline 500 mg. What is the equivalent theophylline dose?

Answer: 500 mg × 0.8 = 400 mg of theophylline.

Clinical pearls & pitfalls

The ATM mnemonic — Aminophylline To theophylline, Multiply — prevents the direction error, which is the main risk in this conversion.
Theophylline is dosed on ideal body weight because it distributes poorly into fat. In an obese patient, dosing on total body weight produces a substantial overdose.
Clearance is affected by a long list of factors: it falls in heart failure, liver disease, and with fluoroquinolones, macrolides, and cimetidine; it rises with smoking, and returns toward baseline over weeks after cessation — a recognised cause of toxicity in newly abstinent smokers.
Toxicity does not follow a reliable sequence. Seizures and arrhythmias can be the first manifestation, particularly in chronic overdose, without preceding nausea.

Assumptions & limitations

The 0.8 factor is a weight-based salt conversion. It does not account for the differing pharmacokinetics of the intravenous and oral formulations.
Does not adjust for the many factors that alter theophylline clearance; conversion is only the first step and concentration monitoring is essential.
Does not cover modified-release oral products, whose dosing intervals differ.

References

Product labelling for aminophylline injection and theophylline extended-release tablets.
Barnes PJ. Theophylline. Am J Respir Crit Care Med. 2013;188(8):901-906.
Hendeles L, Weinberger M. Theophylline: a "state of the art" review. Pharmacotherapy. 1983;3(1):2-44.